The invention is based on a priority application EP 03 293 041.4 which is hereby incorporated by reference.
Wavelength division multiplexing (WDM) is a technique employed in high capacity optical transmission networks to increase their information carrying capacity. WDM systems transmit a plurality of channels, each at a different wavelength, over a single optical fiber. Typically, a WDM system comprises N optical transmitters for optically transmitting N channels of information, a wavelength multiplexer for combining the N channels on one optical fiber, optical amplifiers connected in series by optical fiber cable, a wavelength demultiplexer for separating the optical signal into N channels, and N receivers for detecting the information carried by the N channels. In network configurations having an add-drop multiplexer, the multiplexer is located at a position along the path of the serially connected amplifiers. The add-drop multiplexer is capable of extracting a channel from the network for directing it to an alternative receiver and may also insert into the network a new channel transmitted at the same wavelength as the extracted channel.
Performance of WDM systems is related to the optical signal to noise ratio (OSNR) at the receiver (among other things such as non accumulated non linear effects, or chromatic dispersion). Due to wavelength dependent gain profiles, noise profiles and saturation characteristics of the optical amplifiers and wavelength dependent gain of other components in the optical path, channel OSNR values at the receiver will be unequal for equal transmitter optical power levels. U.S. Pat. No. 5,225,922 Chraplyvy et al discloses a method for equalizing channel performances in point-to-point WDM systems using the optical power level of each channel transmitter and the OSNR measured at each channel receiver. New transmitter optical power levels for each channel are calculated from the optical power level and OSNR values using a specified formula. The channels' transmitters are then set to the new optical power levels. This process is repeated until the difference between the channel's OSNRs is within a predetermined range. Although this method can equalize the OSNR at the receivers within a few iterations, it requires the use of expensive instruments such as an optical spectrum analyzer to measure the OSNRs at the receivers. A further disadvantage of this method is that it is not easily adaptable to more complex optical network configurations such as those having an add-drop multiplexer. It appears that it would be advantageous to provide a less expensive and more flexible method of equalizing channel performance in WDM systems that is capable of equalizing performance in more complex networks such as those having an add-drop multiplexer.
U.S. Pat. No. 6,040,933 discloses a method and apparatus for channel performance equalization in wavelength division multiplexed (WDM) systems. The channel performance is estimated from from the optical power measurement of each transmitted signal. The measurements are taken at the inputs of the first optical amplifiers in the transmission path of the system. The channels are equalized by adjusting the optical power of the channel transmitters. The method is applicable to point-to-point networks as well as more complex network configurations such as those having an add-drop multiplexer (ADM) for inserting and extracting channels from the transmission path. The method can compensate for signals having different bit rates by applying an offset to the amount of optical power adjustment of the channel transmitters.
Furthermore, if different types of optical amplifiers are used in the transmission path, the method can accommodate different noise characteristics of the amplifiers by using their noise figures in determining the amount of optical power adjustment of the transmitters that is required to equalize channel performances.
This method is applicable for a new installation of a system but not for upgrading a system by channel provisioning.
Another way to measure performance of channels is disclosed in U.S. Pat. No. 6,445,471. In this prior art attenuating channels to set a uniform characteristic optimizes the system selected method. The channel's error rate is the relevant criterion to quantify the signal' quality. The measured error rate at the receiver is the relevant value to react on the channels. Bit error measurement is one possibility described. This system needs additional attenuating means and cannot be easily upgraded.